1523 nm is in the infrared range of the optical spectrum. Why do you think that it is too big?

ehild

Because according to my homework set that answer's not right. The wavelengths we're using in class is visible light between 400nm and 700nm and the answer I'm getting from my calculations is in the thousands.

No, I don't think so. the distance between slits (a) multiplied by the sin of the angle given (sinQm) equals wavelength (landa) x (order). In this case, you are refering to dark fringes and the order gets a 1/2 added to it.
So a.sinqm=landa.(5+0.5)
comes out to be 1523 x 10^-9 m.
The equation is okay, but whether or not the values are wrong I can't know, you have the exercise.

Yes all the values I posted are exactly the ones from the exercise. I set up the equation exactly like that too. There must be something really obvious that I'm not getting. Let me do it step by step to see exactly where I'm messsing up.